Helicopter warning system



HELICOPTER WARNING SYSTEM Filed June 2l, 1965 6 Sheets-Sheet 1 ES Gn. dp... -ID .4o

BY aff; 5 Fffmm FrEouENov INPUT .am AT OJQJVEYS Oct. 22, 1968 D. DE LONG ET AL HELICOPTER WARNING SYSTEM 6 Sheets-Sheet 2 Filed June 2l, 1965 N .mi

S R m N @M NGA 0 NM :f Nm r I ER A DF NE O TF. N F EE DJ m w@ Q A 0N Oct. 22, 1968 D. DE vLONG. ET AL 3,407,399 i HELICOPTER WARNING SYSTEM Filed June 21, 1965 6 Sheets-Sheet 5 FIG. 2 *MV-WLV-M` INVENTOR. OEA/TON DELONG JEFF E. FREEMAN ATTORNEYS Oei. 22, 1968 D. DE LONG ET AL HELICOPTER WARNING SYSTEM e Sheets-Sheet 4 Filed June 2l, 1965 uhm-Im mmS moi Arrow/Ys Oct. 22, 1968 D. DE LONG ET AL HELICOPTER WARNING SYSTEM 6 Sheets-Sheet 5 Filed June 21, 1965 voi Oct. 22, 1968 D. DE LONG ET AL HELICOPTER WARNING SYSTEM Filed June 21, 1965 e Sheets-Sheet 6 Fig. 5b

ATTORNEYS United AStates Patent O 3,407,399 HELICOPTER WARNING SYSTEM Denton De Long, Hurst, and Jeff E. Freeman, Fort` Worth,

Tex., assignors to Bell Aerospace Corporation, Wheatl field, N.Y.'

Continuation-impart of application Ser. No. 210,152, July 16, 1962. This application June 21, 1965, Ser. No. 470,311

14 Claims. (Cl.'340263) This invention relates to warning systems for rotating bodies andin particular pertains to an electrical system for providing warning and/or corrective signals in response to an over-speeding or under-speeding of the rotating body, with respect to a predesignated rotating speed range. This application is a continuation-in-part of my copending application, Ser. No. 210,152, tiled July `1.6, 1962, now abandoned.

In the operation of aircraft, malfunctions of the rotating system during flight may result in an over-speeding or under-speeding of the propeller or rotor or other irnportant component which would ultimately result in disaster.

For exam-ple, during helicopter flight there are various malfunctional circumstances lwhich may occur that require immediate corrective procedure on the part of the pilot. In many cases, the only corrective procedure indicated is to start the craft in autorotation and because a substantial degree of altitude is required to make a safe landing, any malfunction which would cause loss kof power or power control should be immediately relayed to the pilot lso that corrective procedures-may be ernbarked upon. For example, during a low power, highy speed letdown, the engine -will normally be operated at zlow rotational speed and light load so that it will not make any discernible noise. Consequently, if the engine ceases to function, the pilot may not become aware ofthis fact until the craft has reached an altitude so that safe autorotation could not be carried out.

As another example, malfunction of the engine governor could cause a dangerous condition and in general, as is stated hereinabove, any circumstances that do not permit the pilot to have full and complete control of the application of power to the craft couldresult in a dangerous situation if the pilot does not become -quickly enough apprised of the. malfunction.y It is, accordingly,

3,407,399 Patented Oct. 22, 1968 ICC FIG. 3 is a block diagram illustrating further princi-ples of the present invention;

FIG. 4 is a block diagram illustrating a still further application of the present invention and illustrating certain modifications thereto; and

FIGS. 5a and 5b are waveforms illustrating certain principles of the invention.

a primary concern of this invention to provide a warning system which willdetect and give a warning in response to a rotational speed of ,either or both the engine and rotor which either exceeds or falls .below a safe operating range. y

More specically, it is an object of this invention to provide apparatus for comparing two electrical pulses, one indicativeof the frequency of the entity being monitored and the other characterized by fixed delay, coincidence. or lack thereof of portions of these two pulses being utilized to provide control over a warning device.

A still further object of this invention is to provide a compound system wherein both the rotor and Vengine will be monitored in accord with the preceding object.

It is another object of this invention to provide a systern as aforesaid wherein the signal from the fixed delay portion is further modified and compared so as to provide a difference signalfor `achieving corrective functions.

. Other objects and advantages of the invention will appear from the description hereinbelow and the accompanying drawing wherein:

FIG. 1 is a block diagram illustrating in general the principles of the present invention;

FIG. 2 is a schematic of the system shown in FIG. 1;

. FIG. .2a-,2l are wave forms at different points in the circuit of FIG. l2;

Referring at this time more particularly-to` FIG. 1, an alternating signal is applied through conductor 10 from a suitable generator driven from either the helicopter engine or rotor, the voltage waveform from thisgenerator at point A being generally as :indicated in FIG. 2a. Part of Athis signal is applied to a'xed delay circuit in'- dicated generally .by the reference character 12 and then to a pulse network general-ly indicated by the :reference character 14. Part of the inputsignal is also applied to a frequency sensitive delay circuit indicated generally by reference characterl and thence-to a pulse network 18. lf the input frequency is within the range to which the various circuits are adjusted, coincidental or overlapping pulses from the networks 14 and 18 will be applied through conductors 20 and 22 to the AND gate 24. When the signals are so present, the AND gate output at conductor 26 will be sutiicient for detection and amplification in the circuit 28 so that the output at conductor 30 of this latter circuit will energize the alarm relay 32 and open the normally closed switch 34 thereof. When, however, the outputs ofthe pulse networks 14 and 18 are not so co-ordinated as to effect energization of relay32, the relay switch 34 will complete the external alarm `circuit through the conductors 36 and 38. Thus, whenever the rotational speed of the associated engine or rotor varies above or below thel predetermined range, a signal will be applied.

The circuit accordingto FIG. 1 has certain advantages in conjunction with helicopter usage. For example, during a low power, high speed letdown, the pilot willnorma'lly be so occupied by the controls he will not have time to monitor the engine tachometer. Consequently, should the engine stop or :fall below its minimum or'idle speed, the pilot will not usually beaware of this fact since the low power application of the engine will not cause -suicient engine "noise as to permit ready detection of its presence, absence or approximate lrotational speed. With the -syS- tem as illustrated in FIG. l, such condition of the engine may be immediately conveyed to the pilot by means of either audible or visible signals, or both, so that he may immediately take corrective measures before it is too l-ate. For example, he may wish to immediately proceed with autorotation for landing.

` The system of FIG. 1 may also be applied to the helicopter rotor so as to give an easily discerniblesignal when the rotor speed falls above or below proper operating range thereof. In some cases, it may be desirable -to utilize the system of FIG. l inI conjunction with both the engine and the rotor and to co-ordinate the signals, in the manner shown in FIG. 3. As illustrated inV this figure, an alternating signal indicative of the speed of the rotor is obtained by a generator 40 driven by the rotor and a similar signalis obtained by an engine driven by generator 42. The signal from generator 40 is applied to the delay and pulse forming network Lttt-and tothe delay network 46 and pulse forming network 48 corresponding to` the blocks 16 and 18 of FIG. 1. Dependent upon the synchronization of the. pulses at conductors 50 and 52, the AND gate 54 will be controlled to produce a pulse signal at conductor 56 extending to the AND gate 58. Similarly, synchronization of the pulses'at conductors 60 -and 62 all control the AND gate 64 whose output at 66 is also applied to an AND gate 58. With the two inputs at 56 and 66 overlapping or in synchronization to the AND gate 58, such AND gateY will, at its output .68, operate the detector and relay 70 corresponding to the circuit 28 and relay 32 of FIG. 1 to produce no signal output at conductors 72 and 74. However, if either the engine or the rotor falls above or"below` its proper operating range, signals will be produced at conductors 72 and 74 yto warn the pilot. Thus, with the compound system as shown in FIG. 3, the system will be sensitive to malfunctions in connection with either or both the engine rotational speed and the rotor rotational speed. 1

The basic circuit configuration of the system according to FIG. 1 is shown in FIG. 2. In this figure, the actuating signalis applied through the conductor 76 to junction A, this signal being ata frequency corresponding to the rotational speed of the eng-ine or rotor, as the case fmay be. The'signal may be produced by'a suitable generator 'driven by the engine or rotor and is of a waveform generally as indicated in FIG. 2a. f 1

v Considering first the upper portion FIG. 2, the incoming signal from junction A is applied to a clipping circuit through the limiting resistor 78, the oppositely poled clipping diodes 80 and 82 -being effective to form Va nearsquare wave as'shown' in FIG. 2b for the junction B. In order t'o obtainwell defined leading and trailing edges for this signal, an amplifier T1, as shown, is used to achieve the square waveform at junction C illustrated in FIG. 2c. The near-square wave is coupled to the amplifier T1 through the capacitor 84 and a voltage divider chain comprising resistors 86 and 88 is connected between a source of positive potential and 4ground potential by conductors 90 and 92, their junction being connected to the base electrode of the NPN transistor as shown. This transistor is connected as a grounded emitter and its collector electrode is connected to the positive potential source through the 4resistor94. The bias current to this transistor is such that itis driven non-conductive during negative portions of the clipped input so that the waveform at junction C exhibits positive pulses coinciding therewith.

The output of the amplifier T1 is applied to the delay circuit comprising the capacitor 96 and resistor 98 whose time constant is in the order of 1A the period of the rnidpoint of the operating frequency range, producing a waveform generally as indicated in FIG. 2d, for junction D. The resistor 98 is adjustable so that, as set fort-h more particularly hereinafter, the lower limit of the frequency range is dictated thereby.

The waveform of junction D is coupled to the amplifier T2 through the capacitor 100 and, as shown, bias current for the base electrode of this transistor is supplied by the resistor chain 102, 104 connected between a positive potential source and ground by conductors 106 and 108. The collector electrode of this transistor is also returned to the positive potential source through the resistor 109 and, as will be apparent from FIG. 2e (junction E), the bias for amplifier T2 is such that during a negative portion of each input pulse, the transistor will be nonconductive, driving the collector electrode positive. A positive spike at the front edge of this pulse (FIG. 2e) and a negative spike at the trailing edge of this pulse is obtained at junction F by means of the dilerentiating circuit comprising the capacitor 110 and the resistor 112, as shown in FIG. 2f. Since the pulse amplifier T3, having its base and collector electrodes connected to a source of positive potential by the resistors 114 and 116 respectively, is biased to produce a positive pulse at junction G in response only to negative input pulses, the duration of each such output pulse will depend upon the RC time constant of elements 110 and 112. This time constant is chosen so as to provide the requisite operating frequency range, as will be presently apparent. The resulting output is illustrated generally in FIG. 2g.

Turning now to the lower portion of FIG. 2, it will he seen that the input signal at junction A is also applied to a phase shifting circuit comprising the capacitor 118 and inductance 120, producing a 90 shift in phase of the input signal. The limiting resistor 122 and oppositely 4 poled clipping diodes 124 and 126 produce a near-square waveform, the waveforms of junction B and C' are similar to, but shifted in phase with respect to, the waveforms of junctions A and B. This near-square clipped signal is applied through'coupling capacitor 128 to transistor T4 which is biased to further square the wave. The base electrode is returned to a source of positive potential at conductor through the resistor 132, the collector electrode is connected to the conductor 130 through the resistor 134and emitter electrode is grounded. The collector electrode of the grounded emitter transistor T5 is connected to the positive potential source through the resistor 136 and its base electrode is connected tothe junction between the resistor 134 and the collector electrode of the transistor T4 so that the transistor T5 forms an inverter producing a waveform at junction I as indicated in FIG. 2]'.

These positive pulses are applied to the differentiating network comprising the capacitor 138 and resistor 140 so that a positive spike is produced at the leading edge of each pulse and a negative spike is produced at the trailing edge of each pulse, as shown in FIG. 2i forthe junction I. It will be noted that for the condition shown in which the input signal at junction A is at the mid-point of the operating range of the system, the negative spikes of FIG. 2i coincide with the positive pulses of FIG. 2g. Thus, by applying the waveform of FIG. 2i to the transistor T6 which is biased to produce positive pulses coinciding with the negative spikes of FIG. 2i,`simultaneous positive pulses will be applied to the collector and base electrodes of the AND gate transistor T7. For this purpose, coupling is achieved by the capacitor 142 and the base and collector electrodes of the grounded emitter pulse amplier T6 are returned to a source of positive potential at conductor 143 through the resistors 144 and 146 respectively. The output at junction I is shown in FIG. 2j.

From the above, it will be apparent that the pulses of FIG. 2j will shift relative to the pulses of FIG. 2g as the frequency of the incoming signal varies. So long as these pulses overlap, the AND gate transistor T, will conduct, the range of overlap being a function of the duration of the pulses of FIG. 2g as achieved by the time constant of the differentiating network 110, 112. The output of the transistor T7 is applied to the base of the transistor T8 connected as an emitter follower and the output across load resistor 148 is applied through the detector diode 150 to a relativelyl large capacitor 152 so that the DC level at junction M will be proportional to the output of the emitter follower output. This voltage is used to drive a conventional amplifier circuit indicated generally by reference character 154 whose output, in turn, is employed to energize the coil 156 of a relay having a normally open switch 158 controlling the alarm circuit.

That is to say, with sufiicient overlap between the pulses of FIGS. 2g and 2j, the relay switch 158 will he open, while cessation of overlap or insuficient overlap between these pulses, as occasioned by 'an input frequency at junction A which is either above or below the operating range, will permit the switch 158 to close and thus complete the warning or alarm circuit.

The resistors 98' and 112, FIG. 2, are shown -to be variable, and this relationship is important as reference 'to FIGS. 5a and 5b will show. In FIG. 5a, the pulse 200 is the output, at G, of the fixed delay circuit, whereas the pulse 202 is the output, at J, of the frequency variable delay circuit. It will he appreciated that the width of the fixed delay pulse 200 will normally be much greater than the Width of the frequency variable pulse 202 and that the' locations, along the time axis, of the leading and trailing edges 204 and 206 of the 'pulse 200 will determine the lower and upper limits of the frequency range. That is to say, for decreasing frequency, the pulse 202 will shift to the left along the time axis until at the lower frequency limit, f1, there is sufficient overlap between t-he pulses 200 and 202 to actuate the AND gate X. At the upper frequency limit, f2, the pulse 202 will shift to the right along the time axis until insufficient overlap exists between the pulses'200 and 202 to actuate the AND gate X.

It frequently happens that the upper and lower frequency limits may not be precisely delineated, or that the limits fora particular aircraft may be altered.`To accommodate, the resistor 98 is first adjusted to shift the pulse 200 along 'the time axisso that its leading edge 204 assumes a new'location vcorresponding to anew lower'frequency limit f3 different from 'theaforem'entioned f1. In FIG. 5a, the pulse 200is shown as having been shifted to the`left, 'so that f3, f1, andis indicated therein by dashed' lines 200. The trailing edge 206 of the' shifted pulse will now, of course, 4define 'some new upper frequency limit f., which, for the condition-"shown, will be less than the aforementioned frequency f2 and will differ therefrom by the same amount 'as f3"differs from f1. Ordinarily, this new upper limit f4 will not correspond to 'the' desired upper limit f5 which may beless than, greater than, or equal to f2. In any event, the resistor 112 is now adjusted to Vary the width ofthe shifted pulse 200 so that its trailing edge 206' is placedat such position, 206", as to correspond to the kdesired upper limit f5. This is shown in FIG. `5b wherein it will be seen that f5 `is greater than f4 but less than f2.AOther conditions for both the lower and upper limits could o f course prevail.

FIGS. 5a and 5b illustrate an important advantage of the present invention, the ability to meet many and variedirequirements of upper'and lower operating'limits. This is an important advantageinasmuch as it allows the warning systems to be adapted to any set of conditions which nay prevail. Further, a pluralityv of systems may be ganged in parallel, as in conjunction with multiple-engine aircraft, While yet assuring that none prematurely operates. n

'Referring now to FIG. 4 wherein alfrnodified system is shown, it will' be' appreciated that the signal input from conductor 76f corresponds tothe signal input in. FIG. 2. Further the blocks I, II IX in FIG. 4 correspond to the' like numbered circuits of FIG. v`2. In the system of FIG. 4,*however,l the output (FIG. 2g) ofthe pulse amplifier V is, in 'addition tobeing applied to AND gate X, applied to 'an RC delay circuit IV which corresponds to the circuit 'IV. Thus, the pulsesof FIG.,2g are differentiated: to produce an outputas shown in" FIG. 2k andthe delay pulse amplifier V', corresponding to the amplifier V, is biased to produce positive pulses in response to the negative differentiated pulses, producing an output as shown in FIG. 2e. Thus, whereasrthe AND gate X in conjunction with the detector XI produces a maximum DC level output for the condition shown in FIGS. 2g and 2j, it will be appreciated that the AND gate XI (corresponding to gate XI) will be non-conductive at this time. Therefore, a maximum voltage difference will appear between the conductors 160 and 162, which voltage difference can be used as feedback to control the entity producing the frequency of the signal at conductor 76'. It will be furthernoted that as the input frequency increases, the gate X and its detector XI will produce a decreasing DC level output while the gate AXI remains non-conductive. On the other hand, as the input frequency decreases, the DC level of detector XI will decrease while the DC level output of the AND gate X and its associated detector XI will increase.

The two detectors XI and XI are connected by means of the isolating diodes 164 and 166 to the warning relay circuit XII so that the system according to FIG. 4 ernbodies the warning function of the system of FIG. 2 as well as embodying the capability for feedback equivalent to frequency error, as aforesaid.

We claim:

1. In a warning system comprising, in combination;

first and second pulse networks connectedto said gate means and adapted to alter the signal output of said gate means in accord with coincidence of signals from such networks;

AC signal generating means adapted for connection to the mechanism being monitored; f f

a fixed delay circuit connected to said AC signal generating means and having its output connected to said first pulse network;

a frequency variable delay circuit connected to said AC signal generating means'and having its output connected to -said second pulse network; said frequency variable delay circuit comprising an LC phase shifting circuit connected to the AC signal -generating means, a pulse forming circuit `connected to the phase shifting circuit and having a positive pulse output in which the trailing edges of the positive pulses coincide with positive peak voltages of the AC signal generating input, a pulse amplifier, and a differentiating circuit connecting the pulse forming circuit to the amplifier, said amplifier being biased to respond only to negative-going input pulses thereto.

2. In a warning system comprising; in'combination;

an alarm circuit including switch means adapted for disposition in dormant and signaling conditions;

gate means having a signal output for determining the condition of said switch means;

first and second pulse networks connected to said gate means and adapted to alter the signal output of said gate means in accord with the coincidence of signals from such networks;

AC signal generating means adapted for connection to the mechanism being monitored;

a fixed delay circuit connected to said AC signal generating means and having its output connected to said first pulse network;

'a frequency variable delay circuit connected to said AC signal generating means and having its' output connected to said second pulse network, said frequency variable delay circuit comprising an LC phase shifting circuit connected to the AC signal generating means, a pulse forming circuit connected `to the phase shifting circuit and having a positive pulse output in which the trailing edges of the positive pulses coincide with positive peak voltages of the AC signal generating input, a pulse amplifier, and a differentiating circuit connecting the pulse forming circuit to the amplifier, said amplifier being biased to respond only to negative-going input pulses thereto, said fixed delay circuit comprising first and second pulse amplifiers, the first amplifier being connected to the input signal through an RC time circuit, the second amplifier being connected to the first amplifier through a differentiating circuit, and both amplifiers being biased to respond only to negative-going input pulses thereto.

3. In a warning system comprising, in combination;

an alarm'circuit including switch means adapted for disposition in dormant and signaling conditions;

gate means having a signal output for determining the condition of said switch means;

first and second pulse networks connected to said gate means and adapted to alter the signal output of said gate means in accord with the coincidence of signals from such networks;

AC signal generating means adapted for connection to the mechanism being monitored;

a fixed delay circuit connected to said AC signal generating means and having its output connected to said first pulse network;

a frequency variable delay circuit connected to said combination;

AC signal generatingmeans and having its output connectedto said second pulse network;V v

second gate means;

a second fixed delay circuit connected to thel output of said first pulse network; l

said second gate means being connected tothe outputs means and adapted to alter the signal output of said t gate means in accord with the coincidence of signals from such networks;

AC signal generating means adaptedfor connection to the helicopter mechanism being monitored;

a fixed delay circuit connected to said AC signal generating means and having its output connected to said first pulse network;`

signal generating means and having its output connected to said second pulse network. 5. In a warning system for an aircraft having a rotary power plant, in combination;

i. a frequency variable delay circuit connected to said AC an alarm circuit including switch means .adapted for 1 disposition in dormant and signaling conditions;

gate means having a signal output for determining the condition of said switch means;

AC signal generating means adapted for connection to the power plant and having asignal output whose frequency is proportional to the speed of rotation of the power plant;

a first pulse network coupled tothe output of said AC signal generating means and having a pulse train output at the frequency of said output of the AC signal generating means and delayed a predetrmined amount with respect thereto;

a second pulse network coupled to the outputof said AC signal generating means and having a pulse train output at the frequency of said output ofthe AC signal generating means and delayed with respect thereto by an amount proportional to such frequency;

the outputs of said first and second pulse networks being connected to said gate means.

.6. In a warning system for an aircraft having a rotary power plant, in combination;

an alarm circuit including an alarm device and gate means for controlling said alarm device;

AC signal generating means adapted to be driven by the power plant and having an output whose frequency is dependent upon the speed of rotation of the power plant;

a first pulse network coupled to the output of said AC signal generating means for producing first output pulses occurring at the frequency of the AC signal, said first pulse network including means for controlling the initiation of saidfirst output pulses in fixed delayed relation to the AC signal independent of the frequency of the AC signal, and means for fixing the termination of said first output pulses independently of the means for controlling their initiations, said first output pulses being coupled to said gate means;

a second pulse network coupled to the output of said AC signal generating means for producing second output pulses occurring at the frequency of the AC signal, said second output pulses being coupled to said gate means; said second pulse network including means for delaying the second output pulses with respect to the AC signal by an amount dependent upon the frequency of such signal whereby at a lower frequency limit f1, as fixed by the initiation o fsaid firstoutput pulses; and .at an upper frequency lirnit f2, as fixed by the termination of .said firstcutput A pulses,- and .at frequencies therebetween,.said first and second output pulses overlap to actuate said gate means; f l .2. 7,A In awarning system for a helicopter having a power plant and a rotordriven by said .power plant, comprising, in combination; t l au Aalarm .circuit including switch .means adapted for dispositionjn dormant and signaling conditions; i Agate means having af signal output fordetermining the condition of saidfswitch means; ,t A. first and secondxpulse networksconnectedto saidugate means and Aadapted to alter thesignal output o f said vgate meanslinaccordnwith the coincidenceof signals from suchy networks; ACsignal generatingmeans driven bysaidpower. plant and having4 an output whose frequency isproportional tothe rotational speed of said power plant;.

a fixed delay-circuit connected to said AC signal generatf ing means ,and having itsoutput connected to said firstpulse network; I l a frequency variable delay circuit connected toisaid AC signal'generating means Vand having its output con.- nectedto said 'second pulsenetwork; i third and fourth 'pulse networks connected to said 'gate lmeans and adapted to alter the signal output thereof in accord with the coincidence of signals from such Lthird and fourth networks;

` second AC signal generating means driven by said rotor y'and' having an output whose Vfrequency is proportional to the rotaryspeed `of said rotor;4 l Aa fixed delay circuit connected to said second vAC sig,- nal generating means and havingV its outputconn'ected to saidthird pulse' network; a frequency`variable delay 'circuit connected to said second AC signal generating means andv having its output'connected to said fourth4 pulse network.'v 8. The system according to claim 7 wherein said. gate means comprises a first and gate connected to the outputs of' said first and secondpulsenetworks, a second and gate connected to the outputsof said third and fourth pulse networks, and a third and gate connected to thefoutputs of the first and second and gates.

9. The ysystem according to claims wherein Asaid sec'- ond pulse network comprises an LC phase shifting circuit coupled "to 'the' AC signal generating means,V a pulse forming circuit coupled to`the phase shifting circuit and having a positive pulse output in which the trailing edges of the positive pulse coincide with positive peak voltages of the AC signal, a pulse amplifier, and a differentiating circuit coupling' the pulse forming circuit to the arnplifier, Lsaid amplifier being biased to respond only to negative-going input pulses thereto.

v1l). The` systemaccording to claim 5 wherein said s econd pulse network comprises an LC phase shifting circuit coupled to the AC signal generating means, apulse forming circuit coupled tothe phase shiftingrcircuit and having a positive pulse output in which the trailing 'edge'sof the positive 'pulses coincide with positive peak voltages of the AC' signal, a pulse amplifier, anda differentiating circuit coupling the pulse forming circuit to the amplifier, said amplifier being biased tofrespond only to'negativegoing input pulses thereto, 'said first pulse networkcomprising first and second pulse amplifiers, the first amplier being coupled to the AC signal through an RC time circuit, the second amplifier being coupled to the fir'sta'mplifier through a differentiating circuit, and both amplifiers being biased to,v respond only to negative-going input pulses thereto. i

11, The system according to claim 10wherein said RC time circuit and the last mentioned differentiating circuit are adjustable to vary their time constants.

12. The system according to claim 6 wherein said second pulse network comprises an LC phase shifting circuit coupled to the AC signal generating means, a pulse forming circuit coupled to the phase shifting circuit and having a positive pulse output in which the trailing edges of the positive pulses coincide with positive peak voltages c'f the AC signal, a pulse amplifier, and a differentiating circuit coupling the pulse forming circuit to the amplifier, said amplifier being biased to respond only to negativegoing input pulses thereto.

13. The system according to claim 6 wherein said second pulse network comprises an LC phase shifting circuit coupled to the AC signal generating means, a pulse forming circuit coupled to the phase shifting circuit and having a positive pulse output in which the trailing edges of the positive pulses coincide with positive peak voltages of the AC signal, a pulse amplifier, and a differentiating circuit coupling the pulse forming circuit to the amplifier, said amplifier being biased to respond only to negativegoing input pulses thereto, said first pulse network comprising first and second pulse amplifiers, the first amplifier being coupled to the AC signal through an RC time circuit, the second amplifier being coupled to the first amplifier through a differentiating circuit, and both amplifiers being biased to respond only to negative-going input pulses thereto.

14. The system according to claim 13 wherein said RC time circuit and the last mentioned differentiating circuit are adjustable to vary their time constants.

References Cited UNITED STATES PATENTS 2,876,004 3/1959 Sink.

2,968,803 1/ 1961 Lindley 340--271 3,010,066 11/1961 Kwast 324-70 3,028,556 4/1962 DuVall.

3,065,461 11/1962 Aronis.

3,098,970 7/1963 Smith 324-70 3,146,432 8/1964 Johnson 340-271 JOHN W. CALDWELL, Primary Examiner.

D. L. TRAFTON, Assistant Examiner. 

1. IN A WARNING SYSTEM COMPRISING, IN COMBINATION; AN ALARM CIRCUIT INCLUDING SWITCH MEANS ADAPTED FOR DISPOSITION IN DORMANT AND SIGNALING CONDITIONS; GATE MEANS HAVING A SIGNAL OUTPUT FOR DETERMINING THE CONDITION OF SAID SWTICH MEANS; FIRST AND SECOND PULSE NETWORKS CONNECTED TO SAID GATE MEANS AND ADAPTED TO ALTER THE SIGNAL OUTPUT OF SAID GATE MEANS IN ACCORD WITH COINCIDENCE OF SIGNALS FROM SUCH NETWORKS; AC SIGNAL GENERATING MEANS ADAPTED FOR CONNECTION TO THE MECHANISM BEING MONITORED; A FIXED DELAY CIRCUIT CONNECTED TO SAID AC SIGNAL GENERATING MEANS AND HAVING ITS OUTPUT CONNECTED TO SAID FIRST PULSE NETWORK; A FREQUENCY VARIABLE DELAY CIRCUIT CONNECTED TO SAID AC SIGNAL GENERATING MEANS AND HAVING ITS OUTPUT CONNECTED TO SAID SECOND PULSE NETWORK, SAID FREQUENCY VARIABLE DELAY CIRCUIT COMPRISING AN LC PHASE SHIFTING CIRCUIT CONNECTED TO THE AC SIGNAL GENERATING MEANS, A PULSE FORMING CIRCUIT CONNECTED TO THE PHASE SHIFTING CIRCUIT AND HAVING A POSITIVE PULSE OUTPUT IN WHICH THE TRAILING EDGES OF THE POSITIVE PULSES COINCIDE WITH POSITIVE PEAK VOLTAGES OF THE AC SIGNAL GENERATING INPUT, A PULSE AMPLIFIER, AND A DIFFERENTIATING CIRCUIT CONNECTING THE PULSE FORMING CIRCUIT TO THE AMPLIFIER, SAID AMPLIFIER BEING BIASED TO RESPOND ONLY TO NEGATIVE-GOING INPUT PULSES THERETO. 